Reproductive outcome and survival of common bottlenose dolphins sampled in Barataria Bay, Louisiana, USA, following theDeepwater Horizonoil spill

Abstract

Common bottlenose dolphins (Tursiops truncatus) inhabit bays, sounds and estuaries across the Gulf of Mexico. Following the Deepwater Horizon oil spill, studies were initiated to assess potential effects on these ecologically important apex predators. A previous study reported disease conditions, including lung disease and impaired stress response, for 32 dolphins that were temporarily captured and given health assessments in Barataria Bay, Louisiana, USA. Ten of the sampled dolphins were determined to be pregnant, with expected due dates the following spring or summer. Here, we report findings after 47 months of follow-up monitoring of those sampled dolphins. Only 20% (95% CI: 2.50–55.6%) of the pregnant dolphins produced viable calves, as compared with a previously reported pregnancy success rate of 83% in a reference population. Fifty-seven per cent of pregnant females that did not successfully produce a calf had been previously diagnosed with moderate–severe lung disease. In addition, the estimated annual survival rate of the sampled cohort was low (86.8%, 95% CI: 80.0–92.7%) as compared with survival rates of 95.1% and 96.2% from two other previously studied bottlenose dolphin populations. Our findings confirm low reproductive success and high mortality in dolphins from a heavily oiled estuary when compared with other populations. Follow-up studies are needed to better understand the potential recovery of dolphins in Barataria Bay and, by extension, other Gulf coastal regions impacted by the spill.

Low reproductive success rates of common bottlenose dolphins Tursiops truncatus in the northern Gulf of Mexico following the Deepwater Horizon disaster (2010-2015)

“…The results from these live animals are consistent with histological studies of dolphins stranded after the spill in the oiled waters of Louisiana, Mississippi, and Alabama, which found significantly higher rates of bacterial pneumonia and abnormally thin adrenal cortices relative to animals outside the region or with individuals collected before the spill (Venn-Watson et al 2015a). Barataria Bay dolphins also showed reproductive failure rates in the first years after the spill 4 times greater than those from a non-oiled stock (Lane et al 2015). The stranding record for some areas also shows indications of impaired reproduction/early life survival, with periods of excessively high rates of perinatal strandings (Carmichael et al 2012, Litz et al 2014, VennWatson et al 2015b.…”

“…In addition to ultrasound, a serum sample was collected from each dolphin and progesterone concentrations were measured at the Animal Health Diagnostic Center (Cornell University, Ithaca, N Y). Pregnancies were classified as (1) confirmed pregnant: a fetus was detected; (2) probable pregnancy: a corpus luteum (CL) with or without uterine fluid was present in conjunction with a serum progesterone value greater than 5 ng ml −1 ; or (3) confirmed not pregnant: the ovary was seen without a CL, progesterone measurements were less than 5 ng ml −1 and the uterus was visualized without an embryo or fetus (Lane et al 2015). 'Confirmed pregnancies' were given estimated due dates based on fetal biparietal skull diameter (Stone et al 1999, Lacave et al 2004, Smith et al 2013.…”

“…For the majority of time in the 4 yr after the drilling rig explosion, dead-stranding rates in areas with the heaviest of oiling exceeded the upper 95% confidence intervals of the historical baseline levels; in some years, stranding rates in certain regions were up to 3.5 to 4 times greater than the 95% confidence levels (Litz et al 2014, Venn-Watson et al 2015b, leading to an unprecedented duration and magnitude in rate of dead dolphin stranding. Moreover, mortality rates in heavily-oiled Barataria Bay, LA, and Mississippi Sound, MS, were approximately 2.5 to 3.5 times higher than those of non-oiled areas, as estimated from the boatbased follow-up surveys of animals examined during capture-release health assessments (CRHA) (Lane et al 2015, Deepwater Horizon Natural Resource Damage Assessment Trustees 2016). The CRHAs of the Barataria Bay and Mississippi Bay animals also revealed a disproportionate number of grossly underweight individuals (Schwacke et al 2014) and a high prevalence of lung disease and evidence of adrenal insufficiency that continued until at least 2014 (Schwacke et al 2014, Smith et al 2017, this Theme Section).…”

Health and Environmental Risk Assessment Project for bottlenose dolphins Tursiops truncatus from the southeastern USA. I. Infectious diseases

“…Health evaluations and pathologic examinations conducted on dolphins from Baratria Bay, Louisiana, demonstrated hypoadrenocorticism, lung disease, and an overall poor prognosis for survival, consistent with the effects of exposure to petroleum products (Schwacke et al 2014). Longer-term follow up has shown decreased reproductive success and survival when compared to the data from Sarasota Bay dolphins (Lane et al 2015).…”

Integrated effects of the Deepwater Horizon oil spill on nearshore ecosystems

“…Dolphins captured and evaluated in 2011 were 5 times more likely to have moderate to severe lung disease when com-pared to Sarasota Bay dolphins (Schwacke et al 2014). Continued monitoring of the Barataria Bay population in 2013, 2014, and 2015 indicated that survival and reproduction were also lower than in the reference population (Lane et al 2015). The combination of injury to vegetation, shoreline stability, and marsh fauna (Table 1) indicated that services the habitat provides to humans and other ecosystem components was likely reduced.…”